(1) Field of the Invention
The present invention relates to a process for the preparation of a perfluorinated copolymer.
(2) Description of the Related Art
A tetrafluoroethylene/perfluoroalkylvinyl ether copolymer (hereinafter referred to as "TFE-PFAVE copolymer") obtained by copolymerizing tetrafluoroethylene (hereinafter referred to as "TFE") with a perfluoroalkylvinyl ether (hereinafter referred to as "PFAVE") is excellent in the melt-moldability over a homopolymer of TFE while retaining high heat resistance and chemical resistance. Accordingly, this copolymer is widely used in various fields, for example, as linings and hoses in the chemical industry, electric wire coverings in the electric and electronic industries, and other molded articles.
As the process for the production of this TFE-PFAVE copolymer, a process in which TFE is merely copolymerized with PFAVE is known at the present (see Japanese Patent Publication No. 48-2223).
However, this process involves the following problems.
In the first place, PFAVE used as the starting material is ordinarily prepared by reacting a dimer of hexafluoropropene oxide with an alkali metal salt and thermally decomposing the reaction product, and this synthesis of PFAVE includes many steps and yields are low at some of these steps. Therefore, the overall yield is low, and PFAVE is expensive.
Furthermore, since PFAVE is poor in the polymerizability, the utilization ratio of PFAVE, that is, the ratio of the units of PFAVE present in the bonded state in the copolymer to the amount of PFAVE charged at the polymerization, is very low. For example, in the process disclosed in Japanese Patent Publication No. 48-2223, the utilization ratio of PFAVE to the charged amount is only several to 30%. Therefore, a special method or step is necessary for recovering expensive PFAVE.
Moreover, the TFE-PFAVE copolymer obtained by the copolymerization of TFE with PFAVE contains at molecule ends --COOH, --CH.sub.2 OH or --COF groups derived from a polymerization initiator or a chain transfer agent or formed by the termination of radicals on the growing chain, and since these terminal groups are thermally unstable and are oxidized or thermally decomposed by heat applied at the molding step to cause generation of hydrogen fluoride or inclusion of bubbles in a molded article. Therefore, corrosion of a molding machine is often caused and the ratio of defective molded articles is increased by inclusion of bubbles.
Still further, a molded article of a TFE-PFAVE copolymer is frequently used for the production of semiconductors where a high purity is required, but problems caused by minute amounts of impurities such as hydrogen fluoride are recently discussed [Proceeding of Annual Technical Meeting of Institute of Environmental Science (32nd meeting), 1986, pages 397-407].
As is apparent from the foregoing description, although the TFE-PFAVE copolymer obtained by copolymerizing TFE with PFAVE have inherently excellent properties such as excellent heat resistance, chemical resistance and electric characteristics, the copolymer involves various problems as mentioned above.